Either use a mage or use a monk, since the best way to kill them is by hitting them a lot of times and not dying, and brute force is useless. I found the sorcerer to be exceptionally good at dealing with them since the damage they take when they hit you shaves off another layer of their death protection.

I have a question related to this quest; I've completed every other quest and bought all the available upgrades, will finishing this quest unlock anything else or is it the end of the current content? Would be good to know before I throw myself against that boss with 50 death protection some more.

Edit - nevermind, I beat him using the standard mystara gnome paladin and answered the question for myself.